We are a hopeless attempt at a blended family. After I split with my ex-husband, we shared care of our two sons. I started a relationship with my current partner (who has two daughters from his previous relationship) a couple of years afterwards and we had a child together.

A few years ago, my younger son from my marriage, then 13, moved in with us full-time. This is when the problems started. My partner has only ever tolerated my sons at best and never has a good word to say about them. He only communicates with them by moaning or criticising, and while they are a handful (the older son is now at university) they really aren't any different from other teenage boys. Last Christmas Eve, my partner came home drunk and was extremely abusive to the boys in front of their brother (now nine) and I've found it hard to move on from that.

It is my second son I am worried about. He says the past five years of his life have been terrible because of my partner. I don't know what to do for the best. He will probably be going to university but it worries me so much that he doesn't have a happy, loving home to return to in the holidays. I suffered in this way myself, coming from a very stressful, difficult home life and hate the thought of him being similarly affected.

Should I leave my partner, taking my youngest son, to try to make a secure base for my middle son to return to? I'm scared I might go to pieces if I leave or that it will adversely affect my youngest son, who is a happy, confident high-achiever at school. But I feel very guilty about my middle son and want in a way to prove to him just how important he is to me and how much I love him. L, via email

I think whatever you do next, you need to do it for yourself. I don't mean in a selfish "stuff my kids" way, but children can deal with separation, even conflict, if it's done in a confident way. What's really hard for them is to see their parents unhappy and doing nothing about it. And what they must never feel is responsible for their parents' actions.

If you could anchor yourself emotionally (and I do recommend some therapeutic help: aft.org.uk) I believe it will be obvious what to do. It was clear from your longer letter that life seems to be something that has happened to you, rather than you seeming to make conscious decisions in. You are not a passenger in your own life.

I consulted a family therapist, and she wanted you to recognise that being a blended family is a complex thing to negotiate; that biological parents can feel drawn to be the mediators and step-parents can feel left out. Plus you all bring with you past histories and hurts. Parenting adolescents isn't easy and not easy for your partner to pick that up when your son was 13 without being able to build full-time on that.

She says to recognise your own ups and downs and how they may have affected your son, and acknowledge how he feels, but also focus on building healthy adult and adult-child relationships. With your sons now moved out, or about to, this could create a good chance for a new type of relationship. Your relationship with your middle son is yours to work on, irrespective of what your partner does. Not that his input isn't important – it is, but you can't change his behaviour, only your own.

It seems the prospect of your son leaving home is crystallising everything. So you may need to let that occasion come and go, and see how you feel when it happens.

The incident when your partner was drunk seems to have been a turning point. "I wonder how he sees it," the therapist asked. "If he felt you understood his struggles to be a better step-parent, would he find it easier to offer a heartfelt apology to the boys or change his relationship with them? I suggest it because it is often hard to change where shame and blame are strong. Whether or not to leave your partner is something for you to consider on its own terms – not to repair something with your son or because of the guilt and regret you feel.

"It is important for your sons not to feel responsible for your decision," she says.